Mailman user unknown / User unknown in virtual mailbox table opensuse

Hi Fellows,
i am having a problem with the mailman integration ...
what works:
+ webinterface within ISPconfig3 creates mailinglists
+ access through the webinterface to the mailinglists works fine
+ mailinvitation gets send to the correspondig address

what doesn't work is sending a mail to the mailinglist-address / as a regular mail or even a subscription request
these mails get a reply notifying the user that the address of the maillinglist is not kown to the system

i already tried to follow some links/posts within the community , but did not have success yet - last one i used was this one

the system is running on opensuse based on the perfect-server howto and the latest version